impr: Added keywords to .desktop file (#1732)

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### Problem description
<!-- Describe the bug that you fixed/feature request that you
implemented, or link to an existing issue describing it -->
This adds a list of strings which may be used in addition to other
metadata to describe the application. This can be useful e.g. to
facilitate searching through entries.

Reference:
https://webcache.googleusercontent.com/search?q=cache:https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html

(The original site is currently down due to a Gitlab upgrade)

These keywords are used by
[Flathub](https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/#launchable),
[KDE
Discover](https://discuss.kde.org/t/does-kde-discover-parse-the-desktop-file-for-categories-and-keywords/7041)
and [Gnome
Software](https://blogs.gnome.org/hughsie/2016/01/07/the-importance-of-keywords-for-the-software-center/).

### Implementation description
<!-- Explain what you did to correct the problem -->
Add a `Keywords` entry to the .desktop file.

### Screenshots
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make before/after sceenshots -->

Here's how the tags are displayed for TeXstudio on its Flathub page: 

![image](https://github.com/WerWolv/ImHex/assets/3226457/35e00272-433e-46cd-9a9e-c42913aefe3f)


### Additional things
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dist/imhex.desktop

@@ -8,3 +8,4 @@ Type=Application
 StartupNotify=true
 Categories=Development;IDE;
 StartupWMClass=imhex
+Keywords=static-analysis;reverse-engineering;disassembler;disassembly;hacking;forensics;hex-editor;cybersecurity;security;binary-analysis;