Move the search bar on top of the file browser for sidebar pages (#3657)

Move the search bar on top of the file browser for the following sidebar
windows:
* "My Projects"
* "My Samples"
* "My Presets"
* "My Home"
* "My Computer"

Add the greyed out text "Search" to the search text edit.
The text is only shown as long as no text is entered in the search field.

Also rename some variable names to something more meaningful. Rename the
member m_l of FileBrowser to m_fileBrowserTreeWidget. Rename the
following local variables in the constructor of FileBrowser:
* ops -> searchWidget
* opl -> searchWidgetLayout

include/FileBrowser.h

@@ -65,7 +65,7 @@ private:
 
 	void addItems( const QString & path );
 
-	FileBrowserTreeWidget * m_l;
+	FileBrowserTreeWidget * m_fileBrowserTreeWidget;
 
 	QLineEdit * m_filterEdit;