Add ability to upgrade tools that Briefcase has downloaded

[freakboy3742]

Briefcase will automatically download (where possible) the tools it needs to build support packages, including:

• WiX
• linuxdeploy
• Java
• The Android SDK

These tools will sometimes put out new releases. However, there is not any way to update the version that Briefcase is using.

The need for this became evident when testing #451. If you have an old version of linuxdeploy, you'll get an error:
Flag could not be matched: --deploy-deps-only. This flag was added on July 14; older versions don't have it.

Describe the solution you'd like

A new command: briefcase upgrade <tool> that allows you to upgrade any tool that has been installed (or install it if it hasn't already been).

A --all option that updates all tools that have already been installed would also be helpful.

Describe alternatives you've considered

Current workaround: manually deleting the tools folder.