Add winprop to select space for new window

[Lythenas]

Fixes #949

This adds a new winprop that allows selecting the space a new window should be inserted.

I added a dropdown to select the workspace. Not sure if that may cause problems with dynamic workspaces etc. Maybe the simple textbox was better. If so, we can simply revert the last commit.

Note: Sometimes it feels a bit confusing because you don't see anything happening if a window opens on another workspace. E.g. on firefox if you drag a tab out of the window to create a new window it is simply gone. But I think that is kind of expected for this. Maybe adding another option to automatically switch to the workspace and focus the window would be good.

[jtaala]

Nice work @Lythenas!

I think activating the window would be the natural/expected behaviour - especially when starting a new app.

[jtaala]

I'm seeing some visual errors on windows that are inserted into a non-active space (i.e. when switching to the space). Might just be due to injecting windows in a way that's a little non-standard.

I'll dig into it a bit on the weekend. Another approach might just be to activate / switch to the space (and make sure it's active) at the insert stage.