Fix saving configurations after deleting some entries

[dforsi]

This PR always deletes all configurations except "General" from the configuration file, otherwise when deleting at least one entry from a group the previous last entry wouldn't be deleted (at least on Linux which uses a .conf file with the ini format). For example if you had configuration-1 and configuration-2 and you deleted one of them (it doesn't matter which one) the old configuration-2 wouldn't get deleted.

At this time the only group that doesn't end with a number is "General" and those which end with a number are:
$ grep startsWith sdrbase/settings/mainsettings.cpp
if (groups[i].startsWith("preset"))
else if (groups[i].startsWith("command"))
else if (groups[i].startsWith("featureset"))
else if (groups[i].startsWith("pluginpreset"))
else if (groups[i].startsWith("configuration"))

The simplest patch is to keep only "General" and delete everything else.
The pros are:

• we don't forget to add groups to delete
• if entries gets deprecated in future, they will be automatically deleted

The cons are:

• if entries gets deprecated in future, they will be automatically deleted :) possibly without informing the user
• it prevents external programs to add their own entries (not sure if it's a valid use case, though)

A more complicated approach would be storing a counter of how many entries were read from the configuration and delete only those exceeding the counter.