Optimize spacing between the AppsDrawer and the right/left panel

[toger5]

things that need to be considered:

• the scrollbar fro the main timeline should not interfere with the resizer between the mainSplit and the right panel
• The margins must be chosen, so that the apps drawer does not overflow the browser window when the right panel is closed.
• The margins should be about 8px

This was also discussed in:
matrix-org/matrix-react-sdk#7112