Improve narrow window UI

[Feichtmeier]

When the window becomes the narrowest app center can become (which is also not very narrow) the navigation takes up so much space that the right pane becomes somehow hard to compose what you can see for example in the searchbar of the installed section:

An alternative can be seen in MusicPod where the navigation moves for narrow windows at a breakpoint out of the left pane...

... and into a Drawer which can be opened by clicking on the sidebar icon:

CC @anasereijo