improve feedback while scanning source roots

after adding a directory to be scanned for source code the directory is scanned while `kcachegrind` blocks. Only the output to console shows any progress. A dialog including a possibility to abort the search (and removing the directory and informing the user about that) would be nice.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: kcachegrind 4:14.12.3-0ubuntu1
Uname: Linux 4.0.1-040001-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.1
Architecture: amd64
CurrentDesktop: Unity
Date: Sun May 31 18:27:46 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (58 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: kcachegrind
SystemImageInfo:
 current build number: 0
 device name:
 channel: daily
 last update: Unknown
UpgradeStatus: No upgrade log present (probably fresh install)