Clarify the meaning of the filter 'uninstallable' in add widgets using i18nc

Give context though i18nc for the use of "uninstallable" so that it won't be misinterpreted as "not installable" by translators, as has happened in some cases (e.g. sv & zh_TW).

This merge request was originally to rephrase "uninstallable" to "removable" for clarity. However, this could potentially also be misunderstood. If there is a way to rename it to something less ambiguous but still accurate, then such a change would be welcome.