Adding cura.sharedmimeinfo

* Originally taken from my own packaging of Cura on launchpad: ppa:thopiekar/cura - https://launchpad.net/~thopiekar/+archive/ubuntu/cura
* It adds mime-types, which are supported by Cura, to the mime-database, so filemanagers can recognize these files.
* This sharemimeinfo includes currently 3MF, STL and OBJ.

cura.sharedmimeinfo

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
+    <mime-type type="application/vnd.ms-3mfdocument">
+        <comment>3D Manufacturing Format Document</comment>
+        <icon name="unknown"/>
+        <glob-deleteall/>
+        <glob pattern="*.3mf"/>
+    </mime-type>
+    <mime-type type="application/sla">
+        <comment>Computer-aided design and manufacturing format</comment>
+        <icon name="unknown"/>
+        <glob-deleteall/>
+        <glob pattern="*.stl"/>
+    </mime-type>
+    <mime-type type="application/prs.wavefront-obj">
+        <sub-class-of type="text/plain"/>
+        <comment>Wavefront 3D Object file</comment>
+        <icon name="unknown"/>
+        <glob-deleteall/>
+        <glob pattern="*.obj"/>
+    </mime-type>
+</mime-info>