Add -transient-window mode

[pijulius]

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Thank you for contributing to rofi! <3

This will add a new mode for the rofi window so when you run it with -transient-window

instead of
-normal-window

it will get the currently focused window and then use that as the parent window and set itself as transient of that window. It is useful so that when you use a keyboard shortcut to start rofi it will always popup on the window you are working on and so it doesn't make you refocus/move your head on large monitors :)

[DaveDavenport]

Good idea!

Can you include it in the manpages? then can will merge it.

[pijulius]

Thanks @DaveDavenport ! happy to see you like it too, not sure about the docs, this is what you had in mind?:
f621786