Improve Layout

This MR is related to issue #733 (closed). It changes the use of the corners like the following graphic shows (top is current, bottom is with this MR)

Be aware, that changing this, it’s not possible anymore to place docks next to the timeline (with the same height), like some users do for example with the "Audio Mixer" (see #648 (closed)). Instead you can rearrange the space on the widget by placing windows in a vertical raw, which leads to a similar result. But it’s not the same!!!