Tables: refactoring of table items

Things that still need to be done
* Replace usage of deprecated methods
* Remove deprecated methods
Merge pull request #65 from stoewer/refactoring

Tables: refactoring of table items
Merge pull request #62 from mpsonntag/filebrowser

Filebrowser

LGTM
TogglePane: parent component for ui switching
MetadataBrowser: toggle between search and nav

However, the search field is not functional for now.
MetadataBrowser: stubs for button actions
MetadataBrowser: import cleanup
ProjectListDialog: swap first and second line in list item
TwoLineListCell: hopefully nicer style
MetadataBrowser: use two line list cells
StatementTableItem: fix value display for non string types
Merge pull request #66 from stoewer/gndata-editor-56-metabrowse

Further improved metadata bowser
FileBrowser: Setting tabs to unclosable
FileBrowser: Refactoring LocalFile methods
Merge branch 'filebrowser' of github.com:mpsonntag/gndata-editor into filebrowser