For reference only (please be aware that this thread is 3 years old)
For anyone that stumbles upon this thread in search of a simple video-cutting-tool and converter
The exact right tool for this job is Avidemux-qt (which I use on a professional basis).
It does exactly what the topicstarter asked for and nothing (at least not much) more.
It is reliable, light, simple, solid, Linux native, GPL and in the standard Ubuntu-MATE repository.
It can do lossless cuts (and joints) and can convert to other formats.
EDIT: I wrongfully declared that Avidemux is in the standard repository which it isn't since several years.
There is, however, an official PPA channel for Avidemux: xtradeb
This is how to add the PPA and install Avidemux:
sudo add-apt-repository ppa:xtradeb/apps
sudo apt update
sudo apt install avidemux-qt