Improve development within Flatpak

[TheEvilSkeleton]

There are a few difficulties with Bottles to test within Flatpak environments. I propose the followings:

• Use .Devel suffix in the app ID (com.usebottles.bottles.Devel) to separate the containers.
• Make utils/install.sh work well inside Flatpak. Currently, it's meant to be used outside of Flatpak, which makes it inconvenient. I changed a bit of the script:

#!/usr/bin/env bash
BUILD_DIR="build/"
if [ -d "$BUILD_DIR" ]; then
	rm -r build
fi
mkdir build
meson --prefix=$PWD/build build
ninja -j$(nproc) -C build
ninja install -C build

This makes it build and install in $(pwd)/build, and can be easily executed by running ./build/bin/bottles. We can run this script by entering in the Flatpak container: flatpak run -d --filesystem=$PWD --command=bash com.usebottles.bottles (or com.usebottles.bottles.Devel), and then running the script: ./utils/install.sh.

• https://docs.usebottles.com/getting-started/installation use org.flatpak.Builder instead, because distros often package it wrong.
• Report GNOME Builder related issues to building Bottles. https://gitlab.gnome.org/GNOME/gnome-builder/-/issues/2061

[orowith2os]

Some initial work for the .Devel change has been pushed here, feel free to use it in the future: https://github.com/bottlesdevs/Bottles/tree/devel-manifest