ENH: Add convenience script to measure the application startup time

For example, on Ubuntu 15.10, 64GB/M.2 PCIe NVMe SSD/Quad Core 3.80GHz.

For a release build, Startup times are the following:

//------------------------
Slicer --no-splash --exit-after-startup
8.641 seconds

Slicer --no-splash --exit-after-startup --disable-builtin-cli-modules
5.663 seconds

Slicer --no-splash --exit-after-startup --disable-builtin-loadable-modules
5.691 seconds

Slicer --no-splash --exit-after-startup --disable-builtin-scripted-loadable-modules
7.107 seconds

Slicer --no-splash --exit-after-startup --disable-builtin-cli-modules --disable-builtin-scripted-loadable-modules
4.017 seconds

Slicer --no-splash --exit-after-startup --disable-modules
1.885 seconds

Slicer --no-splash --exit-after-startup --disable-python
5.727 seconds

Slicer --no-splash --exit-after-startup --disable-python --disable-builtin-cli-modules
3.106 seconds

Slicer --no-splash --exit-after-startup --disable-python --disable-builtin-loadable-modules
3.750 seconds

Slicer --no-splash --exit-after-startup --disable-python --disable-builtin-scripted-loadable-modules
6.139 seconds

Slicer --no-splash --exit-after-startup --disable-python --disable-builtin-cli-modules --disable-builtin-scripted-loadable-modules
3.131 seconds

Slicer --no-splash --exit-after-startup --disable-python --disable-modules
0.838 seconds
//------------------------

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25076 3bd1e089-480b-0410-8dfb-8563597acbee