Allow to specify input purpose

In some situations the type of input is known in advance, one typical thing is PINs for e.g. SIM cards or bank accounts. People then often resort to "visual"/muscle memory to enter the numbers rather than actually remembering the sequence of numbers. Hence it is desirable to be able to display a keypad when a PIN is being prompted for. GTKs text-entry has everything that is needed to forward the input-purpose to the input method (e.g. the on screen keyboard) but gcr's system prompt currently has now way setting that information.

This MR adds support for an input-method property to add support for different input purposes. In contrast to GTK we only use PIN and PASSWORD. Once added we can use it e.g. in g-s-d to set the hint for SIM pin entry in the wwan plugin (gnome-settings-daemon!43 (merged)).

With this we can do:

Closes: #8