Possible improvements to keyboard shortcuts

[astrofrog]

In #1475, @javerbukh implemented an initial framework for keyboard shortcuts. Here are some further things that could be worked on in that direction:

• Currently the keyboard shortcuts that are registered use the Qt key as the first argument to @keyboard_shortcut. It would be better to use a plain string and then in the Qt GlueApplication have a table of correspondence between e.g. 'backspace' and the Qt backspace key (that is, the keyboard shortcut registry should be GUI-framework independent).

• I think the tab keyboard shortcut should be changed to command-tab, as if one has e.g. two viewers and an ipython terminal open, pressing tab will stop cycling through the windows once the IPython terminal is reached, and instead it will start adding tabs in the terminal.

• Similarly I think the delete shortcut should be changed to command-delete, as delete already means something specific when layers or datasets are selected (i.e. remove the layer or dataset)

• We should add some tests for this functionality

• We should add a mention of the addition of keyboard shortcuts to the what's new pages.

• I need to investigate why CI services were timing out with the keyboard shortcuts on by default

cc @javerbukh @brechmos-stsci @drdavella