[Bug] Document build time dependencies when utilising sailfishapp_i18n

[Olf0]

Please let libsailfishapp-devel depend on qt5-qttools-linguist by a Requires: qt5-qttools-linguist in libsailfishapp's spec file, see PR #47.

Alternatively denote in the documentation for libsailfishapp, that if CONFIG += sailfishapp_i18n is specified in a .pro-file, also BuildRequires: qt5-qttools-linguist must be specified to satisfy the build time dependencies.
But the first suggestion (to add a corresponding dependency) would be much easier for developers, as they simply do not have to care, think and remember about this.

• For details see [Bug] TS files are not translated to QM files when building on OBS sailfishos-applications/filecase#68 (comment).
• For further discussion see FSO: Translations not created in OBS.

P.S.: This not being documented did cost me hours of research and tests! Later on I discovered, that others must have also performed this research already, e.g. Petr Tsymbarovich (mentaljam / osetr) for SailfishOS:Chum-GUI.