Fix EAPI 5 gtk3 not in IUSE

This pull already has the changes for mateconf. Please pull this one after  https://github.com/Sabayon/mate-overlay/pull/26.

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EAPI 5 now dies if we do not have GTK3 in IUSE but we do enable it in GCONF2. See example error at the bottom.

Add the neccesasy DEPEND on gtk+:2/:3 to:
app-text/mate-document-viewer-1.5.0
mate-base/mate-desktop-1.5.6
mate-base/mate-file-manager-1.5.{2,3}
mate-base/mate-session-manager-1.5.1
mate-extra/mate-power-manafer-1.5.1

Add these to profile/package.use.mask

Error:
<pre>* ERROR: app-text/mate-document-viewer-1.5.0 failed (configure phase):
*   USE Flag 'gtk3' not in IUSE for app-text/mate-document-viewer-1.5.0
* 
* Call stack:
*          ebuild.sh, line   93:  Called src_configure
*        environment, line 3481:  Called mate_src_configure
*        environment, line 2957:  Called use 'gtk3'
*   phase-helpers.sh, line  219:  Called die
* The specific snippet of code:
*   				die "USE Flag '${u}' not in IUSE for ${CATEGORY}/${PF}"
* </pre>