Add shortcut button for 'current item', 'current experiment' and 'current EExplorer'

[Johan Vallon-Christersson]

If a session keeps track of the most recently viewed Experiment, EExplorer, and Item it would be convenient to have shortcut buttons, equivalent to those in BASE1, to jump to these views.

The buttons should preferably be located in the menu bar, to the left of the Active project text.

When working in BASE you often use links to jump from one view to another, e.g. from an experiment you follow a link to view a Raw bioassay and end up in the Raw bioassays table. To go back to the experiment you jumped from a shortcut in the menu bar would be helpful. Currently you need to go to the experiment table via the menu and the select the experiment. Equivalent examples can be given when jumping from EExplorer or when viewing an item and then browsing tables and menus to find information after which you want to return to the sample you most recently viewed.

[Nicklas Nordborg]

(In [2907]) Fixes #402: Add shortcut button for 'current item', 'current experiment' and 'current EExplorer'