Improve coordination between update prompts for plugins and for JOSM itself

[stanton]

To reproduce:

• Install JOSM.
• Start JOSM and install some plugins.
• Let the setup sit untouched for some two months.
• Start JOSM again.

Observed behavior:

• Splash screen appears and JOSM starts loading.
• Startup is interrupted by a dialog prompting you to update JOSM plugins.
• After clicking the dialog away and finally starting JOSM, you are prompted to update JOSM itself.
• After updating JOSM, you will be promoted to update plugins again (because you just updated JOSM).

Expected behavior:

Only one prompt to update plugins.

Environment:

JOSM 14760 on Ubuntu MATE 18.04

Further information:

There may be multiple approaches to tackle this:

• Simple approach: Check for updates to JOSM before prompting the user to update plugins. If the user chooses to update JOSM, skip the plugin update check until the update is installed. Probably involves the least amount of coding, while already providing an improvement over the current situation.

• Improved approach: If a JOSM update is available and plugins have not been updated in a while, prompt the user what to do: update JOSM, stay on the current JOSM version but update plugins, or start JOSM as-is.

• Perfect approach: Make JOSM capable of updating itself (rather than having the user download the update manually). Pair that with an option to update plugins on next startup, and restart (into the new version) after update, then update plugins (if selected) and restart if needed. Probably involves the most effort but also results in the best UX.