Add support for a horizontal launcher bar

[eliandoran]

This introduces a new setting which allows switching to a new horizontal layout, where:

• The launcher bar is no longer on the left of the screen, but on the top.
• The tab bar also becomes full width.
• The quick search that used to be in the left pane is now part of the launcher bar, and can be manually positioned just like any other launch bar item.
• The "Trilium" icon becomes a simple hamburger menu, aligned to the right on both desktop and mobile.
• On mobile, the current hamburger menu (with two note-specific options) becomes a vertical dot icon similar to a context menu icon. The button is now aligned on the right.

Before:

After:

On mobile, before:

On mobile, after:

[perfectra1n]

LGTM!!!

[meichthys]

This looks nice. I'm looking forward to trying out the mobile version. The sidebar on the mobile version was severely limiting on-screen real estate.

[mm21]

Reminds me of your KDE theme, I like it!