Add desktop and metainfo file for Linux distributions

This adds a desktop file and a metainfo file that are used by Linux
software installation front-ends to present applications.

This will be used by the Flatpak package being put together for
distribution on Flathub:
flathub/flathub#5701

assets/linux/io.github.labbots.NiimPrintX.desktop

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+[Desktop Entry]
+Name=NiimPrintX
+Comment=Create and print labels on Niimbot printers
+Keywords=label;card;print;office;barcode;bluetooth;
+Exec=NiimPrintX.ui
+Icon=io.github.labbots.NiimPrintX
+Terminal=false
+Type=Application
+Categories=Office;
+StartupNotify=true

assets/linux/io.github.labbots.NiimPrintX.metainfo.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop">
+    <id>io.github.labbots.NiimPrintX</id>
+    <launchable type="desktop-id">io.github.labbots.NiimPrintX.desktop</launchable>
+    <provides>
+        <id>io.github.labbots.NiimPrintX.desktop</id>
+    </provides>
+    <name>NiimPrintX</name>
+    <summary>Create and print labels on Niimbot printers</summary>
+    <developer_name>labbots</developer_name>
+    <url type="homepage">https://github.com/labbots/NiimPrintX/</url>
+    <url type="bugtracker">https://github.com/labbots/NiimPrintX/issues/</url>
+    <metadata_license>0BSD</metadata_license>
+    <project_license>GPL-3.0</project_license>
+    <content_rating type="oars-1.1" />
+    <recommends>
+        <control>keyboard</control>
+        <control>pointing</control>
+    </recommends>
+    <screenshots>
+        <screenshot type="default">
+            <image>https://raw.githubusercontent.com/labbots/NiimPrintX/refs/heads/main/assets/linux/main-window.png</image>
+            <caption>Main window</caption>
+        </screenshot>
+    </screenshots>
+    <description>
+      <p>
+        NiimPrintX is an application to design and print labels using NiimBot label printers via Bluetooth.
+      </p>
+    </description>
+    <releases>
+        <release version="0.0.47" date="2024-07-01">
+            <url>https://github.com/labbots/NiimPrintX/releases/tag/v0.0.47</url>
+        </release>
+    </releases>
+</component>