Maintain feature parity with Debian Emacs (i.e., no support for IMAGEMAGICK, LIBOTF and M17N_FLT etc)

From [Run the latest Emacs, even when your OS doesn’t provide one: Introducing Flatpak – Emacs Notes](https://emacsnotes.wordpress.com/2018/11/05/run-the-latest-emacs-even-when-your-os-doesnt-provide-one-introducing-flatpak/)

> At the time of writing, the flathub Emacs
>
> has no support for GPM or IMAGEMAGICK
>
> lacks support for LIBOTF and M17N_FLT
>
> doesn’t support XWIDGETS
>
> Item (2) above means that you may have trouble when editing in Indic, Khmer, Arabic and Farsi.(1) Item (3) is acceptable because it is felt that this feature compromises security(1)

sh-4.4$ diff -u debian-emacs-25.2.2-features.txt flathub-emacs-26.1-features.txt
--- debian-emacs-25.2.2-features.txt 2018-11-05 13:20:10.453834977 +0530
+++ flathub-emacs-26.1-features.txt 2018-11-05 13:21:42.345884892 +0530
@@ -3,16 +3,12 @@
FREETYPE
GIF
GNUTLS
-GPM
GSETTINGS
GTK3
-IMAGEMAGICK
JPEG
LCMS2
-LIBOTF
LIBSELINUX
LIBXML2
-M17N_FLT
NOTIFY
PNG
RSVG
@@ -23,5 +19,4 @@
X11
XFT
XPM
-XWIDGETS
ZLIB

Start by comparing debian-emacs-25.2.2-features.txt with flathub-emacs-26.1-features.txt, focusing on GPM, IMAGEMAGICK, LIBOTF, M17N_FLT, and XWIDGETS. Determine which missing features are intended for the Flatpak and verify the resulting feature list against Debian Emacs; done means the agreed feature parity is restored or the exceptions are documented.

Written by the indexing model from the issue text.