New File Explorer Widget

Add a new control FileExplorerWidget that shows a QFileSystemModel in
QTreeView.
Add file explorer dock widget

Add file explorer dock widget and menu item to the main window.
Merge branch 'feature/file-explorer-#79' into develop

Conflicts:
	app/app.pro
COVERITY-14281: Uninitialized members

Fix uninitialized member 'enabled' in HtmlHighlighter class.
COVERITY-14280: Uninitialized members

Fix uninitialized member 'spellingCheckEnabled' in MarkdownHighlighter
class.
COVERITY-14279: Uninitialized members

Fix multiple uninitialized members in Options class.
Merge branch 'develop' into coverity_scan
Merge branch 'coverity_scan' of https://github.com/cloose/CuteMarkEd into coverity_scan