Support Text in geometry

We should support Text as a geometric element. In its easiest form, the renderer could translate this directly to a call to drawtext. For added compatibility, we should also support styling text as buttons, with all the machinery associated with this. And there is also EditableText to provide a text input field.

• plain text printed
• value substitution for measurements
• docking to points
• docking to window edges and corners
• newlines in static text
• button styling, both pressed and non-pressed
• push button triggers script
• toggle button has accessible state
• editable text shows text field
• text of editable text field accessible from CindyScript
• editable text actually is editable
• reasonable input focus handling

[gagern]

Finally done.

[gagern]

@kortenkamp Since I already did work on the first few points here for #225, it’s only natural that I’m the one to continue that work.