Add a setting to change the searched Thunderbird process name on Windows

[Abestanis]

This setting is only useful on Windows and allows to change the name of the process that is searched to detect if Thunderbird is running. Closes #545.

Changing the setting will take effect when saving, except for if Birdtray already found a process with the old settings. In that case, Birdtray has to be restarted or the currently tracked process has to exit. I think that is acceptable, since this is an advanced feature.

[gyunaev]

See 1ab70f2..a6da1da - its better to hide the settings on a platform where they are unused/have no effect. Otherwise users may end up changing wrong settings and wondering why the change doesn't do anything.

[Abestanis]

That is a good idea! Could you please also hide the Thunderbird window name pattern and Support non-NETWM compliant WMs on Windows, while you are there, if it is not too complicated?

[Abestanis]

Ignore the remark about the Thunderbird window name pattern setting, I see that you have already done it. 😄

[gyunaev]

Done, 1b1c7a5