Enum reference resolving will now search in the @GlobalScope if no class is specified and the enum cannot be resolved in the current class.
Added support for constant references in EditorHelp, e.g.: [constant KEY_ENTER] or [constant Control.FOCUS_CLICK]. It supports enum constants (the enum name must not be included).
Fixes the following Clang 7 warnings:
```
editor/editor_help.h:123:7: warning: 'EditorHelpIndex::popup' hides overloaded virtual function [-Woverloaded-virtual]
editor/editor_help.h:95:7: warning: 'EditorHelpSearch::popup' hides overloaded virtual function [-Woverloaded-virtual]
editor/editor_help.h:96:7: warning: 'EditorHelpSearch::popup' hides overloaded virtual function [-Woverloaded-virtual]
editor/plugins/curve_editor_plugin.h:141:15: warning: 'CurvePreviewGenerator::generate' hides overloaded virtual function [-Woverloaded-virtual]
editor/plugins/script_editor_plugin.h:70:7: warning: 'ScriptEditorQuickOpen::popup' hides overloaded virtual function [-Woverloaded-virtual]
editor/quick_open.h:69:7: warning: 'EditorQuickOpen::popup' hides overloaded virtual function [-Woverloaded-virtual]
main/tests/test_io.cpp:53:15: warning: 'TestIO::TestMainLoop::input_event' hides overloaded virtual function [-Woverloaded-virtual]
servers/audio/effects/audio_effect_record.h:69:15: warning: 'AudioEffectRecordInstance::process_silence' hides overloaded virtual function [-Woverloaded-virtual]
core/os/memory.h:119:2: warning: destructor called on non-final 'ContextGL_X11' that has virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
core/os/memory.h:119:2: warning: destructor called on non-final 'EditorScriptCodeCompletionCache' that has virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
core/os/memory.h:119:2: warning: destructor called on non-final 'Engine' that has virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
core/os/memory.h:119:2: warning: destructor called on non-final 'PhysicalBone::JointData' that has virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
core/os/memory.h:119:2: warning: destructor called on non-final 'VisualServerScene' that has virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
core/os/memory.h:119:2: warning: destructor called on non-final 'VisualServerViewport' that has virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
```
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
- Add pressed state to clear button
- Enable clear button on all inputs with search icon
- Remove duplicate clear buttons
- Fix rendering of icon for center and right alignments
- Add clear button to more search fields
- Add clear icon to default theme
- Add method to control enabled state of clear button
- Add property to enable clear button from inspector
