build: Add a Flatpak manifest

This allows building the master branch of QElectroTech with a simple
command:

$ flatpak-builder _build build-aux/flatpak/org.qelectrotech.QElectroTech.json

build-aux/flatpak/patches/0001-build-Fix-the-installation-paths.patch

@@ -0,0 +1,44 @@
+From 579ee22f1d2bef560ec90d324a0e476b81faf495 Mon Sep 17 00:00:00 2001
+From: Mathieu Bridon <mathieu@hashbang.fr>
+Date: Tue, 7 Jan 2020 14:39:08 +0100
+Subject: [PATCH] build: Fix the installation paths
+
+---
+ qelectrotech.pro | 14 +++++++-------
+ 1 file changed, 7 insertions(+), 7 deletions(-)
+
+diff --git a/qelectrotech.pro b/qelectrotech.pro
+index 07e51fb..2a86b91 100644
+--- a/qelectrotech.pro
++++ b/qelectrotech.pro
+@@ -5,20 +5,20 @@
+ # Chemins utilises pour la compilation et l'installation de QET
+ unix {
+ 	# Chemins UNIX
+-	COMPIL_PREFIX              = '/usr/local/'
+-	INSTALL_PREFIX             = '/usr/local/'
++	COMPIL_PREFIX              = '/app/'
++	INSTALL_PREFIX             = '/app/'
+ 	QET_BINARY_PATH            = 'bin/'
+ 	QET_COMMON_COLLECTION_PATH = 'share/qelectrotech/elements/'
+ 	QET_COMMON_TBT_PATH        = 'share/qelectrotech/titleblocks/'
+ 	QET_LANG_PATH              = 'share/qelectrotech/lang/'
+ 	QET_EXAMPLES_PATH          = 'share/qelectrotech/examples/'
+-	QET_LICENSE_PATH           = 'doc/qelectrotech/'
+-	QET_MIME_XML_PATH          = '../share/mime/application/'
+-	QET_MIME_DESKTOP_PATH      = '../share/mimelnk/application/'
+-	QET_MIME_PACKAGE_PATH      = '../share/mime/packages/'
++	QET_LICENSE_PATH           = 'share/doc/qelectrotech/'
++	QET_MIME_XML_PATH          = 'share/mime/application/'
++	QET_MIME_DESKTOP_PATH      = 'share/mimelnk/application/'
++	QET_MIME_PACKAGE_PATH      = 'share/mime/packages/'
+ 	QET_DESKTOP_PATH           = 'share/applications/'
+ 	QET_ICONS_PATH             = 'share/icons/hicolor/'
+-	QET_MAN_PATH               = 'man/'
++	QET_MAN_PATH               = 'share/man/'
+ 	QET_APPDATA_PATH           = 'share/appdata'
+ }
+ win32 {
+-- 
+2.24.1
+