CheckStyle: Fix IDE compatibility + gitignore

- Fix Eclipse's (and according to Google IntelliJ's) CheckStyle plugins failing on finding the checkstyle_suppressions.xml file. This change does not affect Maven builds.
- Add .checkstyle to the git ignore file.

Author: Pieter12345

.gitignore

@@ -19,4 +19,5 @@ true
 .DS_Store
 /test-backend/
 /nbproject/
-/dependency-reduced-pom.xml
+/dependency-reduced-pom.xml
+.checkstyle

checkstyle.xml

@@ -11,7 +11,7 @@
 
 	<!-- Supression filter to disable specified modules/checks for specified files -->
 	<module name="SuppressionFilter">
-		<property name="file" value="checkstyle_suppressions.xml"/>
+		<property name="file" value="${config_loc}/checkstyle_suppressions.xml"/>
 		<property name="optional" value="false"/>
 	</module>
 

pom.xml

@@ -929,6 +929,7 @@
 							<testSourceDirectories>${project.testCompileSourceRoots}</testSourceDirectories>
 							<includeTestSourceDirectory>true</includeTestSourceDirectory>
 							<configLocation>${basedir}/checkstyle.xml</configLocation>
+							<propertyExpansion>config_loc=${basedir}</propertyExpansion>
 							<encoding>UTF-8</encoding>
 							<consoleOutput>true</consoleOutput>
 							<failsOnError>true</failsOnError>