Move Selectionbar UI setup to .ui file
build: Require next version of vala

So that we can remove workarounds for bug#720825.
Remove workarounds for bug#720825
searchbar: Remove redundant widgets/code
searchbar: Use GtkSearchEntry rather than GdkTaggedEntry

This introduces the regression of first character being ignored if you
start typing while searchbar isn't visible. Hopefully, it will be fixed
by the following commit where we make use of GtkSearchbar as well.
searchbar: Derive from Gtk.SearchBar

This makes use of Boxes.Revealer and Clutter redundant to pack the
searchbar apart from saving us a lot of code.
searchbar: Don't implement UIState

searchbar doesn't exactly reacts to ui state changes so it doesn't need
to implement this interface and it helps us get rid of the now redundant
Clutter actor.
Move Searchbar UI setup to a .ui file

Not much gain at all in this case but lets do this for consistency.
mini-graph: Remove trailing whitespace
Move properties topbar setup to topbar class/UI file
mini-graph: Move UI setup to .ui file

Not much advantage of this on its own but we need to do this to be able
to create MiniGraph instances from other .ui files, which we'll do in a
following patch.
util-app: Remove now redundant two helpers
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