Reading bar

[kjk]

I have started using a reading ruler tool provided by my university and while the software is a pain in the bum and takes over your whole PC, I have found this reading ruler really helps me read papers and reports. I think this feature could really help others with LDs and those who find themselves losing track while reading pdfs.

Originally posted by @AndrewFrazer in #3389

[kjk]

Implement reading bar similar to [Skim on mac] (https://www.youtube.com/watch?v=oZYSi8EdwNM):

• add CmdToggleReadingBar
• add advanced setting ReadingBarBackground - defaults to yellowish or greenish
• is shown in canvas from edge to edge
• can be resized with mouse using top / bottom edges
• can be moved (dragged) with a mouse. if mouse is over it, drag action takes precedence over other actions (text selection, dragging page, interacting with annotations etc.)
• can be closed with CmdToggleReadingBar or by clicking close x button in top right corner, only visible when mouse is over it