Add "slide out" hamburger menu for reading pane

[jzohrab]

The reading pane has quite a few functions: edit text, change mode, stop highlights, etc ... all the hotkeys. Some of these are buried under the help, and as more stuff gets added the list of icons gets longer and the screen more cluttered.

Changing these to a hamburger menu that reveals all the options would be a cleaner page design. The hamburger menu could be to the left of the Lute logo. See e.g. GitHub :

ref a simple design at https://codepen.io/alvarotrigo/pen/xxYGELY - css only (from https://alvarotrigo.com/blog/slide-menus/)

The menu could slide in to the left, over the text, and spell out some of the options, e.g

If we add more things like text resizing, those could perhaps just be added to the menu on the left hand side.

[jzohrab]

Added to the develop branch.