Use ${CMAKE_INSTALL_PREFIX} to provide APP_PREFIX

Currently the value of $prefix is hardcoded in the source code.
This is not a good choice given we have switched to CMake and
that CMake provides the builtin variable ${CMAKE_INSTALL_PREFIX}
to provide this information.

This patch uses the value of ${CMAKE_INSTALL_PREFIX} to populate
flameshot's internal variable APP_PREFIX as well as using it
in generating the org.flameshot.Flameshot.service file with
a proper $prefix value (via CMAKE_INSTALL_FULL_BINDIR as provided
by GNUInstallDirs).

data/dbus/org.flameshot.Flameshot.service.in

@@ -1,3 +1,3 @@
 [D-BUS Service]
 Name=org.flameshot.Flameshot
-Exec=/usr/local/bin/flameshot
+Exec=${CMAKE_INSTALL_FULL_BINDIR}/flameshot

data/dbus/other-path/service-gen.sh

@@ -1,14 +0,0 @@
-#!/usr/bin/bash
-
-DIR=$(pwd)
-TARGET="/usr/bin"
-
-if [ -n "$1" ]
-  then
-    TARGET=${1%/}
-fi
-
-FILE='[D-BUS Service]\nName=org.dharkael.Flameshot\nExec=##REPLACE##/flameshot'
-
-FILE=${FILE//##REPLACE##/$TARGET}
-echo -e $FILE > $DIR/org.dharkael.Flameshot.service

data/dbus/package/org.flameshot.Flameshot.service

@@ -1,3 +0,0 @@
-[D-BUS Service]
-Name=org.flameshot.Flameshot
-Exec=/usr/bin/flameshot

src/CMakeLists.txt

@@ -102,7 +102,7 @@ target_link_libraries(
   Qt5::Network
   Qt5::Widgets)
 
-target_compile_definitions(flameshot PRIVATE APP_PREFIX="/usr")
+target_compile_definitions(flameshot PRIVATE APP_PREFIX="${CMAKE_INSTALL_PREFIX}")
 target_compile_definitions(flameshot PRIVATE APP_VERSION="v0.8.0")
 target_compile_definitions(flameshot PRIVATE IMGUR_CLIENT_ID="313baf0c7b4d3ff")
 target_compile_definitions(flameshot PRIVATE QAPPLICATION_CLASS=QApplication)
@@ -137,8 +137,8 @@ configure_file(${CMAKE_SOURCE_DIR}/docs/bash-completion/flameshot
 configure_file(${CMAKE_SOURCE_DIR}/data/dbus/org.flameshot.Flameshot.xml
                ${CMAKE_CURRENT_BINARY_DIR}/share/dbus-1/interfaces/org.flameshot.Flameshot.xml COPYONLY)
 
-configure_file(${CMAKE_SOURCE_DIR}/data/dbus/package/org.flameshot.Flameshot.service
-               ${CMAKE_CURRENT_BINARY_DIR}/share/dbus-1/services/org.flameshot.Flameshot.service COPYONLY)
+configure_file(${CMAKE_SOURCE_DIR}/data/dbus/org.flameshot.Flameshot.service.in
+               ${CMAKE_CURRENT_BINARY_DIR}/share/dbus-1/services/org.flameshot.Flameshot.service)
 
 # Install Icons
 configure_file(${CMAKE_SOURCE_DIR}/data/img/hicolor/48x48/apps/flameshot.png