[feature] widgets

[jrpie]

Support for widgets would be quite nice. That way users could e.g. add their own weather widget (as explained in #41 I don't want to add weather to µLauncher).

This requires some layout on the homescreen with slots where widgets can be added. Ideas:

• Three slots: top, center (current position of the clock), bottom.
  + very simple
  - looks bad on tablets
• Provide a set of layouts to choose from
  + simple
  - unintuitive
• Grid layout
  + flexible
  - that's what we wanted to get rid of

[jrpie]

In the chat, nefertarimeritmut suggested to add a "widget window", i.e. a slot where multiple widgets can be added.