Whiskermenu settings to xfconf, hide tooltips, double-click distance

This MR contains three changes:

• Move the settings for whiskermenu to Xfconf. Since xfce4-whiskermenu-plugin version 2.8.0 it stores the menu settings in the Xfconf file of the panel (changelog -> 2.8.0 -> "Store settings in Xfconf"). In this version also the default settings of the menu object positions have changed (changelog -> 2.8.0 -> "Replace options for positioning menu layout"). Therefore the new postmarketOS settings look different – but result in the same menu layout as it was before.
• Hide tooltips. They currently don't work well on touchscreens and are disturbing. Unfortunately the option gtk-enable-tooltips to disable them is deprecated. Setting the opacity of the tooltips to zero is a workaround. Changing the font size to zero as well reduces the area covered by the (now transparent) tooltips.
• Increase double-click distance tolerance. This makes double-clicking on touchscreens easier where needed. It shouldn't hurt laptop usage too much.